Spatio-temporally continuous monitoring of surface and ground temperature in Interior Alaska forest by optical Fiber DTS

Abstract:

We have employed an optical Fiber DTS (distributed temperature sensing; N4386B by AP Sensing) system at a taiga site in Interior Alaska in order to monitor the surface and subsurface thermal regime continuously in space and time. The optic fiber cable sensor (multi-mode, GI50/125，dual core; 3.4 mm) of 2.7 km was installed on or below surface, measuring temperature at the half-meter resolution and half-hour interval. The site is in Poker Flat Research Range of the University of Alaska Fairbanks (N 65˚08’, W 147˚26’, 491 m a.s.l), underlain by permafrost. Dominant vegetation is black spruce. Within the area in which the cable was installed, density of spruce trees varies, ranging from open area with mosses to shrubby open forest to closed forest. Measurement was done for two years (from October, 2012 to October, 2014).

When incident photons of a laser pulse is scattered by molecules of optical fiber (SiO₂), a certain amount is back scattered at different frequencies (Stokes and Anti-Stokes peaks). The system detects the intensity ratio of the two peaks of this Raman scattering, which depends on the temperature of the molecules. The distance of the molecules is determines by the time it takes to travel (optical time domain reflectmetry; OTDR).

About 2.0 km of the entire cable sensor lies on the surface to measure horizontal variations of surface temperatures. The diurnal and seasonal components of the variations were analyzed to illustrate their relationship with the overlying canopy characteristics. Cable is also coiled around a PVC tube (outer radius of 4 inch = 10.2 cm) for 120 cm, which is half buried to the ground to measure surface (or snow, when snow-covered) and subsurface temperatures with finer vertical resolution. Five of such tubes were installed in different land cover areas (open and closed forest, shrubs, open area, and relict thermokarst). We will also discuss challenges we encountered during installations and operations.